I haven’t been able to find much research specifically looking at placebo in non-human subjects, though I suspect it’s out there. I do know placebo controls are ubiquitous in research on veterinary therapeutics and effects seem to be about what one would expect in human subjects. Interesting indeed. The quote below is about the only comment I could find specifically addressing the topic, and it doesn’t say much.
Pain Management for the Small Animal Practitioner; Second Edition
William J. Tranquilli, DVM, MS, DACVA; Kurt A. Grimm, DVM, MS, PhD, DACVA, ACVCP; Leigh A. Lamont, DVM, MS, DACVA
The term placebo literally translates to “I shall please.” All proposed mechanisms for the placebo effect suggest an interaction of brain states and somatic health processes (Table 1-1).
When objective and subjective pain assessment scores improve with the administration of a placebo, interpretation of the effectiveness of analgesic treatment is difficult.
Expectancy (a probable mechanism of the placebo effect in humans) requires the ability of the patient to comprehend and anticipate a response to a treatment (e.g., faith or hope). The existence of expectancy in animals is questionable. However, expectancy may lead to observer bias when assessing response to treatment.
Table 1-1. Some proposed mechanisms for the placebo effect in animals.
Human Contact—Visual and tactile contact from a human can cause changes in the subject’s physiologic state, response to painful stimuli, and productivity.
Classical Conditioning—Pavlov first demonstrated classical conditioning with morphine in dogs. Classical conditioning usually requires repeated exposures to the conditioned stimulus and therefore may be a factor in chronic pain treatment.
Endogenous Opioids—This mechanism may explain analgesic placebo responses, but not physiologic responses such as immune modulation or tissue healing.